1. Field of the invention
The present invention relates to a developing unit to develop an electrostatic latent image formed on a latent image carrier in the image-forming apparatus of electrophotography system or electrostatic recording system. More particularly, the present invention relates to an improvement in the developing unit (working with a two-component developer composed of carrier and toner) and a developing roll contained therein.
2. Description of the Related Art
The image-forming apparatus such as copying machines of electrophotography system or electrostatic recording system is customarily equipped with a developing unit to develop the electrostatic latent image formed on a latent image carrier (such as photosensitive drum).
The developing unit of this type operates differently depending on the kind of the developer employed. The developer of two-component type is the most commonest among those which are used for image-forming apparatus, particularly high-speed copying machines. This developer is usually composed of a carrier of iron powder and a resin-based toner. It is held in the developer housing with an opening for development. Opposite the opening is the developing roll, which is a magnet roll having multiple magnetic poles arranged and fixed thereon. The magnet roll is encased in a cylindrical non-magnetic developing sleeve turning around it.
The developing unit of this type mixes the developer (composed of a carrier and a toner) in the developer housing so as to charge the toner by friction. This charging causes the developer to form a magnetic brush of developer on the developing sleeve. The developing sleeve alone is turned so that the developer spreads over its peripheral surface. The developing sleeve carrying the developer is brought into contact with the latent image carrier on which is formed an electrostatic latent image. Thus the toner attaches itself to the electrostatic latent image to turn it into a visible toner image.
Common practice employed for the developing unit of this type is to have the surface of the developing sleeve minutely roughened so that the developer is carried easily. (This procedure is called surface roughening.)
The developing sleeve is made of aluminum or non-magnetic stainless steel. The surface roughening is accomplished mechanically (e.g., sand blasting by a high-velocity jet of hard particles of regular or irregular shape having a certain particle diameter) or electrochemically (e.g., electrolytic etching with pulsating current in an acid solution). See Japanese Patent Laid-open Nos. 250200/1986, 243084/1989, and 132475/1990.
Incidentally, improved image quality and long-life developer, which have been achieved recently, require the copying machines and printers to be more durable mechanically than before. Unfortunately, the developing unit of the type mentioned above has a limited life because its developing sleeve turns while holding the developer (composed of carrier and toner) during copying or printing. In the case of an aluminum developing sleeve, its surface is abraded by the carrier after continued operation. The result is that that the roughened surface of the developing sleeve becomes smooth with time. The developing sleeve with a smooth surface becomes poor in the ability to transport the developer, resulting in thin harsh images.
One way to solve this technical problem is to make the developing sleeve from durable stainless steel; however, this solution is unfavorable to cost saving and weight reduction.
One known way to improve the image quality is to use a toner composed of spherical particles. Such a toner, however, suffers the disadvantage of decreasing in friction with the developing sleeve or fluctuating in the amount of transport due to change in surface roughness with time.